112 THE GEOLOGY OF MINNESOTA. 



[Porphyryte. Calcite and epidote. 



No. 11. PORPHYRYTE. (Diabase.) 



Duluth. East of the elevator; extends along the shore about 800 feet. 

 Ref. Annual Report, ix, page 13. 



Meg. The groundmass of the rock is dark brown to black in color, is aphanitic, 

 and is thickly strewn with reddish porphyritic feldspars. Blotches of epidote are 

 scattered through the rock, and there are spots of epidote and chlorite which perhaps 

 represent amygdules, although it seems most probable that there are no true amyg- 

 dules and that the rock is one of the pseud-amygdaloids described by Pumpelly and 

 Irving. 



Mic. The most prominent feature of the section is the sharply defined plagio- 

 , clase phenocrysts. They have been more or less altered to a micaceous mineral, and 

 sometimes to epidote. The groundmass is composed of small interlacing plagioclase 

 laths, much intigiictite, with homblem/e, chlorite, epidote and perhaps a little qnttiiz. 

 With the exception of the feldspar, and possibly some of the magnetite, all these 

 minerals are secondary. The hornblende is in small grains, usually fibrous, and occurs 

 in the interstices between the feldspars. It evidently is an alteration product from 

 augite, and in places part of the original augite is questionably present. Small apatite 

 needles are rather common. 



One section examined. u. s. G. 



Age. Cabotian. 



Remarks. This rock is very similar to Nos. 7BC and 9. While none of these 

 certainly show augite in the groundmass, there still seems good reason to assume that 

 it was there originally, and so the rocks are called diabase porphyrytes. 



"No. 11 is mainly a massive, homogeneous rock, but in some places finely jointed, 

 so that under the weather it parts into numerous angular blocks. In it are veins 







(near its eastern extension) that seem to cause a greater abundance of the red 

 feldspar crystals in the mass of rock adjoining on either side; * * *"f u. s. G. 



No. 11A. CALCITE AND EPIDOTE. 



Duluth. From a vein in No. 11. 

 Ref. Annual Report, ix, page 13. 



Meg. The calcite and epidote are arranged in rough bands, from one-eighth to 

 one-half inch wide. Some reddish material is mixed with the epidote. 

 No section. 

 Age. In the Cabotian porphyryte. u. s. G. 



No. 11B. GEODE. 



Duluth. From No. 11, which extends about 800 feet along the shore. Some geodes are several feet in 

 diameter, and are rather layers than geodes. 

 Ref. Annual Report, ix, page 13. 



\Sinlh Annual Report, p. 13. 



